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Executing a String from the UNIX Shell Script

Sometimes, in your shell script, you'd want to read in a command--from a file, an user input, etc.--and executing it. It's easy if the command is a simple command that doesn't involve pipe and redirection. But when you involve pipe and redirection, the execution fails. Take the following example script for example:

#!/bin/sh
cmd="cat /etc/hosts.deny | grep \"waldo\""
${cmd}

This is a simple shell script that want to send the /etc/hosts.deny file through the grep function. It's stored in the cmd variable to be executed at a later time. The following is the output of this script:

ALL: ALL
cat: |: No such file or directory
cat: grep: No such file or directory
cat: "waldo": No such file or directory

When executed, cat sends the /etc/hosts.deny file to the stdout and report that "|", "grep", and "waldo" are not found. cat thinks those arguments are file name specifications.

One way to deal with this problem is to write the command string into a temporary executable script and executing it. GetTempPathName.ksh posted elsewhere on this site is useful for this purpose. The following shell script demonstrates this practical example:

#!/bin/sh
cmd="cat /etc/hosts.deny | grep \"waldo\""
tempExec=`GetTempPathName.ksh "waldo"`
echo "${cmd}" > "${tempExec}"
chmod 700 "${tempExec}"
"${tempExec}"
rm "${tempExec}"

Chieh Cheng
Tue, 27 Feb 2007 16:32:08 -0800

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